Photonic crystals (or devices) are two dimensional or three dimensional systems with special optical characteristics that originate from periodic structures. The periodic modulation of refractive index gives rise to a photonic band gap in which electromagnetic radiation within a certain wavelength band are totally reflected. This leads to electromagnetic radiation within a certain wavelength being selectively diffracted or refracted, causing interference coloring.
Shape memory polymers are polymer materials which may be returned from a deformed state to their original shape via an external stimulus. The external stimulus typically is temperature in the case of thermally-activated shape memory polymers but can also be the application of an electric or magnetic field, light or a change in pH.